The exploration for hydrocarbons and minerals at present is becoming of widespread importance, as the earth's resources are being consumed at an alarming rate. The seismic exploration method has proved very successful over the last 65 years for locating hydrocarbons. Today there is a great demand to adapt these techniques to the search for minerals, which places greater demands on the processing of data.
The basic principle of seismic exploration consists of transmitting a sound wave through the earth's rock structures and picking up reflected and/or refracted waves. Analysing the properties of these sound waves yields information about the earth's geological features. These features, together with samples taken from exploratory bore holes, enable the geologist to determine the position and extent of hydrocarbon and mineral deposits.
In land exploration, a large number of geophones are spaced apart along a line extending over many kilometers, in either one or both directions, from a sound source. In a typical land data acquisition arrangement, the geophones are each connected to a recording truck via independent channels, which may consist of multiconductor cables carrying analogue signals, or a single cable carrying a digital multiplexed signal from each geophone. At the recording truck the signals from each geophone channel are connected to a switching device which selects which channels are to be recorded. The sound source e.g. an explosive charge, is moved along the line at fixed intervals of distance, and the geophone channels selected for recording are those in the proximity of this sound source. At each interval or "shot point" a recording is made and in this manner data is acquired while moving along the exploration line.
In marine exploration the reflected and refracted sound waves are detected using a hydrophone streamer which is towed in a straight line behind a survey ship. The streamer consists of a number of sections, each containing a hydrophone (or group of hydrophones), and are linked to the recording equipment via independent channels. The ship moves along the exploration line towing the sound source, which produces a sound wave at fixed intervals of distance. The sound wave passes through the water into the earth's surface and the reflected and refracted waves are detected by the hydrophone streamer and recorded. In this manner data is acquired while the survey ship moves along the exploration line.
At present, digital recording is used almost exclusively in seismic exploration. This gives a large dynamic range, and signal processing techniques can be applied to the data directly using digital computers. Typical recording equipment used at present has a capacity of up to 120 channels, sampling rates in the range 0.25 to 4 milliseconds, and a dynamic range of 84 dB. The analogue-to-digital conversion generally consists of 14 bits for the magnitude, one bit for the sign and three bits to represent the gain applied to the amplifier during conversion. Data from each geophone is sampled sequentially, digitised and placed onto tape (i.e. in a multiplexed form).
As sound sources, explosives are used extensively for both land and marine exploration, but are now beginning to be replaced by other sources. For example, a weight drop method is often used in land exploration, in which a weight is dropped from a predetermined height onto the ground to produce the sound wave. The weight can be dropped many times at a particular location, and the reflected seismic signals detected by the geophones for each drop can be added to increase the signal amplitude. A method called vibroseis is frequently used for land exploration in which a mass in contact with the ground is made to vibrate with a frequency that is continuously changed or swept over a certain range. The swept frequency sound wave travels into the earth's surface and is reflected and refracted. Since a swept frequency of long duration is used, as opposed to a single shock or pulse, the detail from each reflecting structure is blurred. The detail is recovered using a process called correlation, which compresses the swept frequency waveform into a pulse. Thus, the signal from each geophone is cross-correlated with the signal or reference signal applied to the vibrating mass. The result is a record of the type obtained had a pulse been sent through the earth's surface. The vibroseis method has the advantage that there is control over the frequencies that are transmitted.
The ultimate aim of seismic data processing is to produce an image (a section) that is representative of geological features. In practice the final section that is produced is not a perfect representation of rock strata, and it has to be subjected to interpretation by experienced geologists and geophysicists.
The type of processing applied to the seismic data is very much dependent upon the type of exploration and the geology of the area. For example the exploration for oil, gas, coal and minerals requires different processing.
At present the infield processing of seismic data signals from the geophones is limited to filtering, noise rejection, summing and correlation. Portable data acquisition systems are known, which can be hand-carried into jungle regions or mounted in small trucks for desert use. Arrangements for summing of seismic data used with the weight drop method of exploration, and typical vibroseis data acquisition arrangement, which will perform infield correlation and summing, are large and can only be used mounted in large trucks. The summer and correlator are purpose built units and cannot be adapted to perform other functions. These various systems and arrangements provide data from the various channels recorded in multiplexed form on tapes.
The recorded data tapes from the field are sent to processing centres, where large main frame computers perform all the processing requirements to produce the seismic sections. The field tapes are first demultiplexed, which results in all the samples for channel 1 being separated for the first sound source position (shot point), followed by all the samples for channel 2, and so on. This is repeated for each shot point. The data is changed back from the geophones-sampled-sequentially form to channel-sequential form. This process on a VAX main frame computer is very time consuming and could be in terms of hours for a typical day's field data.
The second stage of processing is concerned with signal extraction and signal-to-noise enhancement. It is at this stage that noisy channels and spikes on the data are edited out. Data can also be summed or cross correlated, if this has not already been carried out in the field.
The third stage is geometric corrections. Survey data is placed into the computer at this point to provide corrections for the earth's terrain variations. Also included at this stage is sound velocity information, this being determined from refraction data.
The fourth and final stage is concerned with data enhancements, and it is at this point that very sophisticated techniques can be employed. The data can be filtered in two dimensions and can be used for the removal of noise generated by ground roll, which is a slow moving wave travelling in a shallow region (or "weathering layer") just below the earth's surface. The waveform is reshaped, using deconvolution, and compensates for changes in the shape of the sound wave as it passed through the rock layers.
A process called stacking is almost universally carried out which reduces the noise from multiple reflections. Trace amplitude equilization can also be applied to compensate for the weaker signals furthest from the sound source, time variable filtering to compensate for the attenuation at high frequencies, migration to transform the apparent reflection positions to true positions, which is useful near regions of geological faulting, and many other processes can be used, the final result being the required seismic section.
At present, data acquisition equipment tends to be in the form of separate units, each purpose built, and bolted together to form a recording system. Often the units come from different manufacturers, requiring appropriate interfacing circuitry to be built. This results in greater requirements for spare parts in the field, and greater expertise in personnel to repair the equipment if it should fail.
Over the years there has been a demand for more channels to be recorded. This has led to smaller identical units being added together to achieve greater channel capacity. These units were originally designed for independent use and using them in this way tends to lead to technical problems, such as longer sampling rates. The units cannot be bolted together in unlimited numbers.